1. Technical Field
The present inventions relate generally to measuring the contents of a mixture, and more specifically to measuring the constituent contents of a wet gas at any point along the gas pipeline where a measurement is required including subsea and onshore.
2. Description of Related Art
Crude oil usually comprises a mixture of elements, often including petrol, water, and various gasses. A determination of constituent contents in crude oil is required to measure accurately net volumes of actual oil in sales, taxation, exchanges, and custody transfers. Tankers are now transporting liquefied products to ports of call, which would also require measurement. Main pipelines are exchanging products between different owners, and therefore, the contents need to be measured. The water content of crude oil is also significant because it can cause corrosion of equipment and problems in processing. Thus, various methods have been developed for measuring the contents of crude oil.
Background: Wet Gas Measurement in Subsea Pipelines
Wet gas metering covers a variety of measurements in production streams with high to very high gas volume fractions. There is a need for the direct measurement of gas under these conditions in such applications as gas condensate and high GOR (gas oil ratio) fields as well as many production operations where gas from separation systems may contain liquid. More gas will be produced in the future from remote and subsea fields where production, capital investment, and operating costs must be optimized. For example, real time measurement of gas and liquid flow rate are critical in a subsea production system, which will improve well allocation, optimize reservoir production, and enhance flow assurance. (SPE 77351, “Wet Gas Metering: Trends in Applications and Technical Developments”, Mehdizadeh et al., Society of Petroleum Engineers, Inc., 2002, which is hereby incorporated by reference.)
Offshore lines coming to shore also need to be measured because they are required to have a low water content to prevent hydrate formation in the line.
Background: Standard Wet Gas Flow Meters
The difficulty in measuring wet gas arises from the fact that gas and liquid are both fluids with different properties. In a standard flow meter, the measured pressure of a flowing fluid can be used to determine its velocity, from which its volumetric flow rate is calculated. If the fluid is all gas or all liquid, the differential pressure accurately reflects the flow rate. With a mixture, however, there are no distinctions between the two, which results in uncertain measurement of both fluids. To determine the individual flow rates, the ratio of gas to liquid must be known.
The inaccurate reading is a result of the design of the standard flow meter. The basic flow meter consists of a pipe that is constricted on one end. The constriction causes the fluid flowing through the pipe to accelerate. Before and after the constriction; two pressure measurements of the accelerating liquid are taken and the difference in pressure is converted into a volumetric flow rate.
Background: Aluminum Oxide for Moisture Adsorption
The use of aluminum oxide for moisture adsorption is well known in the industry. The surface attracts and retains water molecules by association with the bonds. Since this is a weak attraction, there is a point at which the absorption and desorption reaches an equilibrium with the surrounding moisture content. Moisture measurements have been made with capacitance measurements using a very thin aluminum oxide surface with imbedded electrodes. When the water is absorbed, the capacitance changes, and therefore, a measurement is made. This surface must be thin in order to allow the water molecules to accumulate in a region where the electrical field is present. This thin region usually has gold or other metal flashed onto the surface to implement a conductivity or capacitance measurement. This metalization also allows contaminants such as ethylene glycol and methanol to become trapped between the metal and the substrate that biases the measurement.
It is well known to electrical engineers generally (and particularly to microwave engineers) that the frequency of an RF (radio frequency) oscillator can be “pulled” (i.e. shifted from the frequency of oscillation which would be seen if the oscillator were coupled to an ideal impedance-matched pure resistance), if the oscillator sees an impedance which is different from the ideal matched impedance. Thus, a varying load impedance may cause the oscillator frequency to shift.
To help explain the use of the load-pull effect in the disclosed innovations, the electromagnetics of a dielectric-loaded transmission line will first be reviewed. If a transmission line is (electrically) loaded with a dielectric material, changes in the composition of the dielectric material may cause electrical changes in the properties of the line. In particular, the impedance of the line, and the phase velocity of wave propagation in the line, may change.
This can be most readily illustrated by first considering propagation of a plane wave in free space. The propagation of a time-harmonic plane wave (of frequency f) in a uniform material will satisfy the reduced wave equation:(∇2+k2)E=(∇2+k2)H=0,where
E is the electric field (vector),
H is the magnetic field (vector), and
∇2 represents the sum of second partial derivatives along the three spatial axes.
This equation can be solved to define the electric field vector E, at any point r and time t, asE(r,t)=E0 exp[i(k·r−ωt)],where
k is a wave propagation vector which points in the direction of propagation and has a magnitude equal to the wave number k, and
ω=Angular Frequency=2 πf.
In a vacuum, the wave number k has a value “k0” which is
                                             K            0                    =                    ⁢                      ω            /            c                                                                    =                        ⁢                                          ω                ⁡                                  (                                                            μ                      0                                        ⁢                                          ɛ                      0                                                        )                                                            1                /                2                                              ,                    where
ρ0=Magnetic Permeability of vacuum (4π×10−7 Henrys per meter),
ε0=Electric Permittivity of vacuum ((1/36π)×10−9 Farads per meter), and
c=Speed of light=(ρ0ε0)−1/2=2.998×108 meters/second.
However, in a dielectric material, the wave number k is not equal to k0; instead
                                 k          =                    ⁢                      ω            /                          (                                                c                  ⁡                                      (                                                                  μ                        r                                            ⁢                                              ɛ                        r                                                              )                                                                    1                  /                  2                                            )                                                                                =                        ⁢                                          ω                ⁡                                  (                                                            μ                      0                                        ⁢                                          μ                      r                                        ⁢                                          ɛ                      0                                        ⁢                                          ɛ                      r                                                        )                                                            1                /                2                                              ,                    where                μr=Relative Permeability of the material (normalized to the permeability μ0 of a vacuum), and        εr=Relative Permittivity of the material (normalized to the permittivity ε0 of a vacuum).        
Thus, if the relative permeability μr and/or the relative permittivity εr vary, the wave number k and the wave propagation vector k will also vary, and this variation will typically affect the load pulled oscillator frequency.
Frequency Hopping in a Load-Pulled Oscillator
In a typical free-running oscillator, the oscillator frequency is defined by a resonant feedback circuit (the “tank” circuit), and can also be pulled slightly by a reactive load, as noted above. Thus, such an oscillator can be broadly tuned by including a varactor in the tank circuit.
As the oscillator's frequency is thus shifted, the phase difference between the energy incident on and reflected from the load element (which is preferably a shorted transmission line segment) will change. This phase difference will be equal to an exact multiple of 180° at any frequency where the electrical length of the transmission line segment is an exact multiple of λ/4.
As the oscillator frequency passes through such a frequency (i.e. one where the transmission line segment's electrical length is equal to a multiple of λ/4), the load's net impedance will change from inductive to capacitive (or vice versa). As this occurs, the frequency of the oscillator may change abruptly rather than smoothly. This jump in frequency will be referred to as a frequency “hop”.
For a transmission line of length 1 which contains a dielectric material of relative dielectric constant εr, the frequency at which one full wavelength (1λ) exists in the transmission line is equal to c (the speed of light in vacuum, which is 2.995×108 meters/second) divided by the length of the line in meters and by the square root of the relative dielectric constant of the material:
Frequency1λ=c/(lεr1/2).
For example, for a one-foot-long line filled with a material having εr=1, l=12 inches (=0.3048 meters), and
Frequency1λ=(2.995×108)/(0.3048×1.0)≈980 MHz.
However, since one wavelength actually contains two excursions from inductive to capacitive reactive impedances, only one-half wavelength is required to see one frequency hop of the load pulled oscillator. If the transmission line is terminated into a short or an open, the resulting effective length is increased to twice the actual length, since a standing wave is generated (due to the energy incident at the short or open being reflected back to the input of the transmission line). In essence, the energy travels down the line, gets reflected, and travels back to the input. With this taken into account, the first frequency with a wavelength long enough to cause a frequency “hop” of the oscillator is one fourth the length calculated above, or 245 MHz.
Multiples of this first quarter-wavelength frequency will also cause the impedance seen at the input to the transmission line to go from inductive to capacitive reactance. The longer the transmission line, the greater the number of phase transitions that will occur. Longer line length also multiplies the phase changes that are brought about by a change in the dielectric constant. For every one-quarter wavelength change in the effective (electrical) length of the line, the complex impedance seen at the oscillator changes by 180°.
For example, suppose that a given oscillator, coupled into a low loss load with an electrical length of one-quarter wavelength (λ/4), provides typically 50 MHz of load pulling frequency change (total excursion through all phases −λ/4 would have 180 degrees). If the monitored material changes enough to produce a change of only one degree of phase in the electrical length of the load, the oscillator frequency will change by 50 MHz divided by 180 degrees=277.78 kHz per one degree. This represents an absolute resolution of 3.6×10−6 degrees of phase change for each Hertz of sensitivity. For every additional quarter wavelength of line length, this sensitivity to phase is multiplied by 1.5. This is due to the change in phase being an additive function of every additional quarter wave in the measurement section.
In a typical tuning frequency versus voltage plot for a VCO (voltage controlled oscillator) loaded into a shorted transmission line, the height of the “hop” can be measured by holding the VCO tuning voltage constant, while a transmission line terminated into a short is varied in electrical length to cause a full rotation of the impedance vector seen at the VCO's input port. The resulting data of frequency versus length of the transmission line will show a jump in frequency (a delta frequency from the bottom of the “hop” to the top of the “hop”) which coincides with the delta frequency of the “hop” seen when the VCO was swept using the tuning voltage.
Thus, if the VCO is swept across a frequency band and the number of frequency “hops” was counted, the number of “hops” reveals the number of wavelengths in the transmission line.
This provides a means for determination of the range of dielectric constant change in a medium even when it rotates the phase vector multiple times (and therefore, the oscillator frequency returns to the same value multiple times). If the dielectric constant of the material in the transmission line is increased, then the above equations show that the frequency of the first full wavelength is decreased by the square root of the dielectric constant. Additionally, this means that the number of wavelengths at a fixed frequency increases with increasing dielectric constant. These facts imply that the VCO tuning curve will see more “hops” as the dielectric constant is increased due to the increasing fraction or whole wavelengths encountered.
Ideally, the oscillator will not cease oscillations (or break into multiple frequency oscillation or spectral breakup) into any load regardless of the load characteristics. However, this is not a strictly necessary condition for use of the disclosed method and system innovations.
Measurement of Substances with a High Microwave Loss Factor
A measure of the dielectric loss of a material is typically given as the dielectric loss tangent (a unit-less parameter) which is defined as the tangent of the imaginary part divided by the real part of the complex dielectric constant. Low loss materials are typically below a loss tangent equal to or less than 0.01. When the disclosed systems are used to measure materials with a high loss factor, the material's absorption begins to dominate the load versus frequency effects, but a measurement capability still exists due to the sensitivity of the load pulling method.
Additional information, which can be obtained from conventional microwave measurement methods and also from load pulled measurement, is covered below.
Difference in Operation Frequency
Additional information can be obtained by making another measurement at a much higher frequency. Since materials change properties versus frequency, the amount of frequency change will vary versus the frequency of operation.
A VCO will typically be designed to cover approximately one octave above its turn on frequency. If a VCO would not give enough frequency change to see the desired range of varying parameters versus operating frequency, additional oscillators, which run at any frequency required to obtain appropriate data, can be used and may be switched into the coaxial line.
When two widely spaced frequencies are measured for a medium under study, the difference (delta) frequency between these two measurements will be unique for a given medium. This phenomenon will aid in distinguishing constituents and the progress of mixing or reaction.
Monitoring of Insertion Loss
If the incident power and the reflected power are measured in a system where the final load is a short, the difference in powers will be twice the insertion loss of the medium (since two transits occur through the medium of interest). The insertion loss measurement will aid in determination of the changing conductivity of the medium or its change in absorption of the RF energy. This information can be related to the mixing or reaction products to further distinguish unique situations. Also a transmission measurement can be implemented where the through loss is measured by power incident on the measurement section and after the measurement section. This is through conventional microwave measurement means.
Effect of Complex Permeability
The magnetic permeability μr can also be dynamically measured by the disclosed techniques. Since the velocity varies with (μrεr)−1/2, changes in μr will change the phase shift through a given physical length of line, and thus change the frequency of the oscillator.
A sample-containing waveguide, like that of the principally preferred embodiment, will typically have locations where the electric field is strong but the magnetic field is zero; at such locations only permittivity will affect the oscillator load pull frequency. However, there will also commonly be locations in a waveguide where the magnetic fields are locally strong and electric field is zero: at these locations, only the permeability will affect the propagation characteristics of the transmission line (and, therefore, contribute to the oscillator frequency for a load pulled oscillator or would contribute to the phase and amplitude measurement of a conventional microwave measurement system).
A system can be built to sample (primarily) one of these parameters. For example, to sample the permeability, the coaxial transmission line will be terminated into a short where the medium of interest is located only in close proximity to the short. A waveguide structure supports very well defined electrical and magnetic field functions, and the sample can be suitably placed in such a structure to measure primarily the permeability.
Typical compounds and substances do not have varying magnetic permeabilities and therefore, most of the discussion will involve the changing complex permittivity. However, the effects of changing complex permeability will create similar changes in the oscillator load pulling characteristics or in the conventional microwave system phase and amplitude. If a substance such as barium titanate is studied, the effect of the changing permeability must be considered along with the change in permittivity unless the system is designed specifically to measure only one of these.
Coupling the Active Device in a Load Pulled Oscillator
An unusual feature of the oscillator configuration used with the present invention is the separation of the load of interest from the resonant circuit proper. The configuration used isolates the two through the active device. It is the non-linear behavior of the transistor that provides the changes in frequency as the load is changed. The loop gain of an oscillator must be unity with an appropriate phase shift to cancel the negative impedance's imaginary part around the resonant loop. The initial gain of the active device must be greater than unity before oscillations can begin in order for the oscillator to be self starting. This extra gain is reduced to unity by the saturation of the active device upon establishment of the oscillations. Saturation of a device normally also changes the phase shift through the device. This requires a change in the operation frequency as the load changes due to the shift in loop gain and phase by the saturated condition change in the active device.
Spectral Purity of Oscillator in a Load Pulled Oscillator
It has been discovered that, in a system using a free-running oscillator as described above, spectral purity of the oscillator is an important concern. Many microwave oscillators exhibit “spectral breakup”, wherein the spectrum of the oscillator's output actually contains multiple frequencies. In most microwave oscillators, this is not a problem since a tuned feedback element will be used to stabilize the gain element, and/or isolation or buffering stages are used to prevent the oscillator's feedback loop from being perturbed by extraneous resonances. However, in a load-pulled system, since such buffer stages are not used, spectral purity turns out to be quite important. For example, a spurious resonance in the feedback loop (e.g. due to a low-quality RF choke, or due to two impedance mismatches) can permit the oscillator to hop to a frequency which is determined (at least partly) by a harmonic of the spurious resonance, in which case the degree to which the oscillator frequency has been pulled by the changing load will be obscured.
To avoid such problems in a load-pulled system, a small series resistor can be interposed in the RF output of the oscillator, before the measurement section connection. This resistor adds a small amount of damping, which helps to suppress oscillation at secondary frequencies.
To further improve stability, a shunt resistor can be attached to the RF output of the load-pulled oscillator. This resistor adds to stability, by fixing a maximum magnitude for the load impedance seen at the RF output line.
Background: Other Approaches to Electrical Characterization
Various types of apparatus have been proposed for measuring the concentration of one substance in another, particularly the concentration of a liquid or flowable substance in another liquid or flowable substance. Various devices that utilize the broad concept of determining composition of matter by measuring changes in a microwave signal are disclosed in U.S. Pat. No. 3,498,112 to Howard; U.S. Pat. No. 3,693,079 to Walker; U.S. Pat. No. 4,206,399 to Fitzky et al.; U.S. Pat. No. 4,311,957 to Hewitt et al.; U.S. Pat. No. 4,361,801 to Meyer et al.; U.S. Pat. No. 4,240,028 to Davis Jr.; U.S. Pat. No. 4,352,288 to Paap et al.; U.S. Pat. No. 4,499,418 to Helms et al.; and U.S. Pat. No. 4,367,440 and U.S. Pat. No. 4,429,273, both to Mazzagatti; all of which are hereby incorporated by reference.
U.S. Pat. No. 6,593,753 to Scott et al. teaches a planar probe with two transmission lines. However, this particular innovation detects the differences in chemical interactions and is directed towards characterizing a single phase (such as an all liquid stream) stream. It is not designed to measure a multi-phase stream such as wet gas (moisture in gas, natural gas, natural gas liquids and free water) and does not take into account factors such as the pressure and temperature of the substance being measured, which become important in multiphase stream analysis. This particular innovation was also designed to be used with a load-pulled oscillator. By contrast, the present inventions disclose methods and systems for determining the amount of water, petrol, and gas in a multi-phase, wet gas stream. This determination can be made with any standard microwave method and is not limited to load-pulled oscillators.
U.S. Pat. No. 6,810,719 to Dutton et. al. relates to a flow measurement system that comprises a separator, a Coriolis flow meter, and a water cut monitor. This patent also appears to employ calculations that require iterative solutions. The method disclosed herein provides an additional variable by which the solution for the equations is simplified greatly. Also by contrast, the present innovations do not require a separator or flow meter although a Coriolis flow meter providing density to aid in the solution of the equations would be beneficial. Typically, Coriolis flow meters cease working at some point when the flowing medium of gas and liquids becomes non-homogeneous. The present innovations also provide a method for calculating the amount of petrol, water, and gas in a wet gas that is less computationally intensive than U.S. Pat. No. 6,810,719.
Wet Gas Measurement System
The present inventions relate to measuring the constituent contents of a wet gas. It is preferably implemented at any point along the gas pipeline where a measurement is required including subsea and onshore.
The present inventions determine the total permittivity of a wet gas and the amount of water in the wet gas. These data, combined with some assumptions about the system, permit calculation of the amount of petrol in the mixture.
In one preferred embodiment, the present inventions comprise two independent probes for identifying the amount of petrol, water, and gas in a wet gas. The probes each have one or more openings that allow the materials of the flow to penetrate and enter the probes.
The first probe allows passage of the flow of the mixture into the chamber of the probe. The permittivity of the mixture inside the chamber of the probe is measured. This probe provides the overall permittivity of the entire mixture.
The second probe, preferably immediately downstream of the first probe, is configured with a microwave system and includes aluminum oxide or similar bed of material so that the total water content of the flow is measured. The aluminum oxide or similar material is chosen so that it attracts and holds water in proportion to the water content in the flow. As the moisture content in the flow changes, the proportion of water inside the probe, and hence the measured permittivity, will change. Thus, the second probe filled with alumina or other material provides understanding of the permittivity of the water portion of the mixture in the flow.
This information, combined with the overall permittivity of the gas, oil, and water as obtained from the first probe, allows for the determination of the amount of petrol and the amount of gas in the wet gas mixture.